#1Hot
Bangkok
- Day
- 30.9°C
- Sea
- 29.4°C
- Sun
- 1.5h
- Rain days
- 26
Your September weather guide
Weather in Thailand in September
September is a reasonable month for Thailand if you can work around frequent rain, but a poor fit for a holiday built around reliable sunshine. Average daytime temperatures reach 29°C, with 25 rainy days and just 1.6 hours of sunshine. Koh Samui has the fewest rainy days among these destinations, making it the better coastal bet on that measure. The catch is that even there, you should plan for wet weather rather than treat it as an interruption.
Long-term climate averages, not a forecast · 1991-2020
- Average daytime high
- 29.0°Cacross 6 destinations
- Average night-time low
- 24.8°Cafter dark
- Sea temperature
- 29.0°Cwarm enough to swim
- Rainy days
- 25days1 mm or more
Thailand, month by month
Turn up the warmth
Warmest places in Thailand in September
Ranked by average daytime high. The full comparison is in the table below.
#2Hot
Chiang Mai
- Day
- 29.2°C
- Sea
- -
- Sun
- 1.8h
- Rain days
- 24
#3Hot
Koh Samui
- Day
- 28.9°C
- Sea
- 29.1°C
- Sun
- 1.6h
- Rain days
- 23
#4Hot
Krabi
- Day
- 28.6°C
- Sea
- 28.9°C
- Sun
- 1.5h
- Rain days
- 26
#5Hot
Khao Lak
- Day
- 28.3°C
- Sea
- 28.5°C
- Sun
- 1.3h
- Rain days
- 27
#6Hot
Phuket
- Day
- 28.0°C
- Sea
- 28.9°C
- Sun
- 1.6h
- Rain days
- 25
Thailand destinations ranked for September
All 6 places we track in Thailand, ordered by daytime high.
Scroll sideways for more weather, down for more places. Click a heading to sort.
| # | Destination | ||||||
|---|---|---|---|---|---|---|---|
| 1 | Bangkok | 30.9°C | 24.9°C | 29.4°C | 1.5 h | 26 | 12 h 42 min |
| 2 | Chiang Mai | 29.2°C | 22.9°C | - | 1.8 h | 24 | 12 h |
| 3 | Koh Samui | 28.9°C | 26.3°C | 29.1°C | 1.6 h | 23 | 13 h 6 min |
| 4 | Krabi | 28.6°C | 24.4°C | 28.9°C | 1.5 h | 26 | 13 h 12 min |
| 5 | Khao Lak | 28.3°C | 24.4°C | 28.5°C | 1.3 h | 27 | 13 h 6 min |
| 6 | Phuket | 28.0°C | 25.6°C | 28.9°C | 1.6 h | 25 | 13 h 6 min |
6 destinations shownClimate averages, not a forecast.
September climate overview
Daytime highs across Thailand in September run from 28.0°C (Phuket) to 30.9°C (Bangkok). In Bangkok about 100% of September days reach 25°C. Nights average 24.8°C; the coldest 10% of nights in Phuket fall to 24°C or below.
Rainfall in September
Expect around 25 rainy days (1 mm or more) in September, with Koh Samui the driest (23 days) and Khao Lak the wettest (27 days). The wettest September at Bangkok between 1991 and 2020 was 2012 with 530 mm; the driest was 2001 with 139 mm.
Sea temperature
The sea averages 29.0°C around Thailand in September - warmest at Bangkok (29.4°C). That is comfortable for swimming.
Sunshine and daylight
Around 1.6 hours of sunshine a day on average, out of 12.1 hours of daylight (sunshine estimated from cloud cover).
September against the rest of the year in Thailand
September is the 9th warmest month of the twelve in Thailand. It is 0.3°C cooler than August and 0.3°C cooler in October. The hottest month here is April (32°C) and the coolest December (29°C).
Which part of Thailand in September?
- Most sunshine: Chiang Mai, 1.8 h a day.
- Driest: Koh Samui, about 23 rainy days.
- Warmest sea: Bangkok, 29°C.
- Hottest: Bangkok, 31°C by day.
- Coolest (better for walking and cities): Phuket, 28°C.
What to pack for Thailand in September
Based on the averages: summer clothes for the day; a rain jacket - roughly 25 wet days; swimwear - the sea is 29°C.
Similar weather elsewhere in September
Within 1.5°C of Thailand's September daytime average: Seville (30°C), Limassol (30°C, sea 27°C), Puerto Vallarta (30°C, sea 30°C), Ho Chi Minh City (30°C), Antalya (30°C, sea 28°C). See the full ranking for September.
Which coast makes most sense?
Koh Samui is the strongest coastal choice here if avoiding rain is your priority, though its 23 rainy days hardly make it a dry option. Daytime and sea temperatures both average 29°C, so warmth is not the limiting factor for a beach holiday. Krabi has the same daytime and sea temperatures but 26 rainy days, giving you less reason to choose it purely for the weather. Phuket is slightly cooler by day at 28°C, with 25 rainy days. Khao Lak has 27 rainy days and a sea temperature of 28°C. For any of these destinations, keep beach plans flexible: warm water does not guarantee suitable swimming conditions, and dependable sun is not September's selling point.
Heat, evenings and practical planning
Bangkok is the hottest option at 31°C by day, so rain should not be mistaken for cool weather. Light clothing and somewhere sheltered to pause are a more useful combination than heavy waterproof layers. Chiang Mai averages 29°C by day and 23°C at night, making it a better fit if you prefer less heat after dark. Its 24 rainy days still call for flexible plans, rather than a schedule dependent on staying outdoors. Coastal evenings remain warm too, with nights averaging 26°C in Koh Samui and Phuket. Pack breathable clothes, a compact umbrella and footwear you can manage when wet. Leave room to move outdoor time around the forecast, and take breaks from the heat, particularly in Bangkok.
A little more detail
Thailand in September FAQs
Plain answers, grounded in the averages.
Where is the hottest place in Thailand in September?
Bangkok is the warmest of the 6 places we track in Thailand in September with an average high of 30.9°C; Phuket is the coolest at 28.0°C.
Is the sea warm in Thailand in September?
The sea averages 22°C or more at Bangkok (29°C), Koh Samui (29°C), Krabi (29°C), Khao Lak (29°C), Phuket (29°C) in September.
How much warmer is Thailand than London in September?
Thailand averages 29°C by day, compared with London's 19°C. Expect to dress for heat despite the frequent rain. Bangkok is hotter still at 31°C, so it is not a cool-weather alternative to a coastal stay.
Is the sea warm enough for swimming?
Sea temperatures average 29°C in Koh Samui, Krabi and Phuket, and 28°C in Khao Lak. That makes water temperature unlikely to be the obstacle. Decide when to swim based on local conditions and safety advice, not warmth alone.
Where has the coolest nights?
Chiang Mai has the coolest nights among these destinations, averaging 23°C. Krabi and Khao Lak average 24°C, while Bangkok averages 25°C. Koh Samui and Phuket stay warmer at 26°C, making them less appealing if you prefer cooler evenings.
About these averages
ERA5-Land climate reanalysis via Open-Meteo (11 km grid), 1991-2020
Averages computed from daily values at 6 destination points.Records are the highest and lowest daily values in that 30-year period, not all-time records. These are climate averages, not a forecast for your trip. How we measure holiday weather
- Hot · 25°C+
- Warm · 20°C to below 25°C
- Mild · 15°C to below 20°C
- Cool · below 15°C